Chitosan–Alginate Gels for Sorption of Hazardous Materials: The Effect of Chemical Composition and Physical State

Chitosan, alginate, and chitosan–alginate (50:50) mixed hydrogels were prepared by freeze casting, freeze-drying, and subsequent physical cross-linking. Chitosan was cross-linked with citrate and alginate with calcium ions, while the mixed gels were cross-linked with both cross-linking agents. Both cryogels and xerogels were obtained by lyophilization and drying of the hydrogels. We investigated the effect of the chemical composition and the physical state of gels on the gel structure and sorption of model dyes. Alginate and mixed gels cross-linked with Ca2+ ions sorbed 80–95% of cationic dye from the solutions. The chitosan gels are primarily capable of adsorbing anionic dyes, but at near-neutral pH, their capacity is lower than that of alginate gels, showing 50–60% dye sorption. In the case of alginate gels, the dye sorption capacity of xerogels, cryogels, and hydrogels was the same, but for chitosan gels, the hydrogels adsorbed slightly less dye than the dried gels.

Int. J. Mol.Sci.2024, 25, x FOR PEER REVIEW structure.Supercritical extraction as a drying method usually produces an aerogel hydrogel without damaging the gel network.The aerogel has a particularly low d and thermal conductivity.In the case of xerogels, drying at room temperature can the pores to collapse due to the high capillary pressure caused by solvent removal.F drying of cryogels, on the other hand, often leads to cracks and large pores, which explained by the crystallization of water (or other solvents) during freezing [9].Hydrogels can be produced from hydrophilic chitosan and alginate under rel mild reaction conditions using inexpensive cross-linking agents [2,36].Chitosan pared by deacetylation of chitin.Since deacetylation is never complete, chitosan considered a copolymer composed of β-(1-4)-D-glucosamine and β-(1-4)-N-acetyl- Polymer hydrogels are 3D-structured cross-linked systems containing large amounts of water in their pores.The hydrogel structure prepared from polymer solution can be achieved by various methods (phase separation-induced by pH, temperature or nonsolvent, cryogelation, freeze-thaw process, physical or chemical cross-linking) [2,6,9,[36][37][38][39]. Various drying methods can be used to produce xero, aero-, and cryogels from polymer hydrogels [2,6,9,39].For xerogels, the gel is dried slowly at room temperature.Generally, the resulting gel has high porosity, a large surface area, and a small pore size [39,40].Cryogels are freeze-dried to form a super-macroporous net-structure.Despite their porous structure, they have the advantage of good osmotic and mechanical stability [40].In the case of aerogels, the solvent is replaced by a gas without collapsing the gel structure.Supercritical extraction as a drying method usually produces an aerogel from a hydrogel without damaging the gel network.The aerogel has a particularly low density and thermal conductivity.In the case of xerogels, drying at room temperature can cause the pores to collapse due to the high capillary pressure caused by solvent removal.Freeze-drying of cryogels, on the other hand, often leads to cracks and large pores, which can be explained by the crystallization of water (or other solvents) during freezing [9].
Hydrogels can be produced from hydrophilic chitosan and alginate under relatively mild reaction conditions using inexpensive cross-linking agents [2,36].Chitosan is prepared by deacetylation of chitin.Since deacetylation is never complete, chitosan can be considered a copolymer composed of β-(1-4)-D-glucosamine and β-(1-4)-N-acetyl-D-glucosamine units.In an acidic medium, the amine groups of chitosan are protonated, imparting a positive charge to the polymer.The alginate is a linear copolymer containing β-D-mannuronic acid (M) and α-L-guluronic acid (G) units.These elements can form a block copolymer (GGGGG or MMMMM) or an alternating copolymer (GMGMGM).The carboxylic groups in alginate are deprotonated in neutral or alkaline media, and the polymer becomes negatively charged.Chitosan is easily cross-linked with citric acid and alginate with Ca 2+ ions [2,3,6] in aqueous media (Figure 2).Furthermore, the polyelectrolyte produced from the two polymers can also be cross-linked with these chemicals.Thus, the gels obtained by physical cross-linking do not contain harmful substances and can be used in various applications.Hydrogels can be produced from hydrophilic chitosan and alginat mild reaction conditions using inexpensive cross-linking agents [2,36] pared by deacetylation of chitin.Since deacetylation is never complete considered a copolymer composed of β-(1-4)-D-glucosamine and β-(1-4 cosamine units.In an acidic medium, the amine groups of chitosan ar parting a positive charge to the polymer.The alginate is a linear copoly D-mannuronic acid (M) and α-L-guluronic acid (G) units.These element copolymer (GGGGG or MMMMM) or an alternating copolymer (GMG boxylic groups in alginate are deprotonated in neutral or alkaline media becomes negatively charged.Chitosan is easily cross-linked with citric with Ca 2+ ions [2,3,6] in aqueous media (Figure 2).Furthermore, the po duced from the two polymers can also be cross-linked with these chemic obtained by physical cross-linking do not contain harmful substances a various applications.Chitosan and alginate are often used to prepare hydro-, cryo-, or aerogels, and microor nanoparticles or nanofibers containing absorbents, which can bind dyes and metal ions [17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35]41,42].Adsorbents based on alginate are mainly used for cationic dyes, while chitosan-based gels bind with all types of dyes, often in combination with other polymers or fillers [41].In many publications, the authors investigate the kinetics of sorption and give the sorption capacity of the adsorbents.These values are often compared with those shown in other publications to indicate how suitable the adsorbent produced in a particular work is.However, the data comparison is only useful if the circumstances under which the sorption test was performed (pH, adsorbent mass, dye solution concentration, adsorption time) are provided.Zhao et al., [24] for example, prepared chitosan-alginate gels with different compositions using a similar method to ours and studied the dye sorption of the gels.Their work investigated the dye uptake kinetics and pH dependence of several different gel compositions, among others.Finally, they found that their sample could bind with nearly 1500 mg/g of methylene blue, which is a good value compared to the reference values given in their table.However, to compare the efficiency of one's sample with that of the above samples, one needs to perform lengthy browsing and calculation to do so, because it is difficult to figure out from the presented curves and tables strictly which sample the given value applies to, and what amount of adsorbent, starting solution volume, and dye concentration were used.In the absence of these, the dye removal efficiency cannot be calculated, even though it indicates what percentage of the dye content of the solution can be removed with the given adsorbent.Similar problems are also found in most of the available literature.Table 1 shows the results of some adsorption data published in the literature.They allow a comparison of the sorption efficiency of different alginate-and chitosan-based adsorbents.
In many publications, it is difficult to find under which conditions the specific adsorption capacities were measured.Hence, the data in Table 1 need to be completed and are sometimes uncertain (indicated by a question mark).We have only focused on the data for adsorbents made of alginate and/or chitosan.As most publications have investigated the effect of complex systems, we include the composition of the adsorbent tested in brackets.The data in Table 1 show that the dye removal efficiency of the pure polysaccharide adsorbents is mostly less than 90%, which, of course, may be because the contact time is often very short.
A review of the publications also revealed that the comparison of dye uptake of gels in different physical states (i.e., hydro-, cryo-, and aerogels) has yet to be investigated [43] despite this knowledge's practical and theoretical importance.In our research, we prepared chitosan, alginate, and chitosan-alginate hybrid hydrogels without filler and other polymers using environmentally friendly methods of freeze casting and freeze-drying followed by physical cross-linking.We prepared xerogels and cryogels from the hydrogels and investigated the structure and dye-sorption capacity of the different gels.

Morphology of Gels
The structure of the lyophilized non-cross-linked samples is shown in Figure 3.It can be seen that a highly porous, sponge-like structure has formed after freezing and lyophilization.While interconnected pores can be observed in the pure chitosan sample (Figure 3a), the structure of the pure alginate without cross-linking (Figure 3b) is layered and lamellar.The properties, typical of both pure materials, can be observed in the 50-50% samples (Figure 3c).For hydrogels, a similar but slightly looser structure can be assumed.
be seen that a highly porous, sponge-like structure has formed after freezing and lization.While interconnected pores can be observed in the pure chitosan sample 3a), the structure of the pure alginate without cross-linking (Figure 3b) is layered mellar.The properties, typical of both pure materials, can be observed in the 50-50 ples (Figure 3c).For hydrogels, a similar but slightly looser structure can be assum The structures of the cryo-and xerogels obtained after cross-linking and dry shown in Figures 4 and 5, respectively.The SEM images reveal that the structure cryogels is similar to that of the lyophilized, non-cross-linked samples (Figure 4), b regular than that of the pure chitosan and alginate gels.On the other hand, the st of the mixed cryogels (Figure 4c,d) depends significantly on the cross-linking agen Similar observations can be made for xerogels (Figure 5), but their structure is eve irregular and solid.The structures of the cryo-and xerogels obtained after cross-linking and drying are shown in Figures 4 and 5, respectively.The SEM images reveal that the structure of the cryogels is similar to that of the lyophilized, non-cross-linked samples (Figure 4), but less regular than that of the pure chitosan and alginate gels.On the other hand, the structure of the mixed cryogels (Figure 4c,d) depends significantly on the cross-linking agent used.Similar observations can be made for xerogels (Figure 5), but their structure is even more irregular and solid.

Swelling of Gels
Although FT-IR spectroscopy is an excellent method for monitoring the structure of polymer systems [44,45], it is not suitable for studying the structure of hydrogels due to the large amount of bound water, so swelling tests checked the cross-linking of the samples.When cross-linking is insufficient, no gel is formed, and the samples dissolve in water instead of swelling.Figure 6 shows the different degrees of gel swelling.The swelling values of mixed gels are also presented, cross-linked with citrate and Ca ions.The results show that all four gels have significant swelling.The alginate gel swells slightly more than the chitosan gel, and the mixed gel cross-linked with Ca ions shows more significant swelling than the one cross-linked with citrate ions.This suggests that citrate ions form a more compact gel structure than Ca ions.

Swelling of Gels
Although FT-IR spectroscopy is an excellent method for monitoring the structure of polymer systems [44,45], it is not suitable for studying the structure of hydrogels due to the large amount of bound water, so swelling tests checked the cross-linking of the samples.When cross-linking is insufficient, no gel is formed, and the samples dissolve in water instead of swelling.Figure 6 shows the different degrees of gel swelling.The swelling values of mixed gels are also presented, cross-linked with citrate and Ca ions.The results show that all four gels have significant swelling.The alginate gel swells slightly more than the chitosan gel, and the mixed gel cross-linked with Ca ions shows more significant swelling than the one cross-linked with citrate ions.This suggests that citrate ions form a more compact gel structure than Ca ions.

Dye Sorption of the Gels
The dye uptake of chitosan, alginate, and mixture gels was investigated in methylene blue and methyl orange solutions at different concentrations.Figure 7 illustrates the color of the residual hydrogels obtained after testing in dye solutions with a concentration of 40 mg/L.The dye uptake of the cationic methylene blue and anionic methyl orange dyes (Figures 8 and 9, respectively) at 300 mg/L dye concentration is plotted as a function of time for the cryo-and xerogels.
While the alginate gel predominantly binds with the positively charged methylene blue, the chitosan gel binds with the negatively charged methyl orange.The mixed gels cross-linked with Ca 2+ ions strongly bound to both dyes, but their methylene blue sorption (~80%) was much higher than the methyl orange sorption.Surprisingly, the methyl orange sorption capacity of the xerogels exceeded the dye uptake of chitosan.Further investigation is required to explain the results.The mixed gels cross-linked with citrate could only bind with methyl orange with an efficiency of nearly 40%.

Dye Sorption of the Gels
The dye uptake of chitosan, alginate, and mixture gels was investigated in methylene blue and methyl orange solutions at different concentrations.Figure 7 illustrates the color of the residual hydrogels obtained after testing in dye solutions with a concentration of 40 mg/L.The dye uptake of the cationic methylene blue and anionic methyl orange dyes (Figures 8 and 9, respectively) at 300 mg/L dye concentration is plotted as a function of time for the cryo-and xerogels.
While the alginate gel predominantly binds with the positively charged methylene blue, the chitosan gel binds with the negatively charged methyl orange.The mixed gels cross-linked with Ca 2+ ions strongly bound to both dyes, but their methylene blue sorption (~80%) was much higher than the methyl orange sorption.Surprisingly, the methyl orange sorption capacity of the xerogels exceeded the dye uptake of chitosan.Further investigation is required to explain the results.The mixed gels cross-linked with citrate could only bind with methyl orange with an efficiency of nearly 40%.
While the alginate cryo-and xerogels can adsorb nearly 95% of the dye in the methylene blue solutions, the chitosan gels adsorb methyl orange with an efficiency of only around 60% (Figures 8 and 9).The pH values of the methylene blue and methyl orange solutions were 8.34 and 7.82, respectively.These pH values confirm the measured adsorption values.In the alkaline methylene blue solution, the free carboxyl groups of alginate readily interact with the positively charged dye.However, the slightly alkaline pH of the methyl orange solution does not contribute to the significant protonation of amine groups in chitosan and the strong interaction with the negatively charged methyl orange molecules.While the alginate cryo-and xerogels can adsorb nearly 95% of the dye in the methylene blue solutions, the chitosan gels adsorb methyl orange with an efficiency of only around 60% (Figures 8 and 9).The pH values of the methylene blue and methyl orange solutions were 8.34 and 7.82, respectively.These pH values confirm the measured adsorption values.In the alkaline methylene blue solution, the free carboxyl groups of alginate readily interact with the positively charged dye.However, the slightly alkaline pH of the methyl orange solution does not contribute to the significant protonation of amine groups in chitosan and the strong interaction with the negatively charged methyl orange molecules.While the alginate cryo-and xerogels can adsorb nearly 95% of the dye in the methylene blue solutions, the chitosan gels adsorb methyl orange with an efficiency of only around 60% (Figures 8 and 9).The pH values of the methylene blue and methyl orange solutions were 8.34 and 7.82, respectively.These pH values confirm the measured adsorption values.In the alkaline methylene blue solution, the free carboxyl groups of alginate readily interact with the positively charged dye.However, the slightly alkaline pH of the methyl orange solution does not contribute to the significant protonation of amine groups in chitosan and the strong interaction with the negatively charged methyl orange molecules.

Effect of the Physical State of Gels on the Dye Sorption
It is obvious that the dried cryo-and xerogels can adsorb dyes almost equally (Figures 8 and 9).We were curious to compare the dye uptake of the hydrogels with that of the dried gels, so we also performed dye uptake studies with the gels obtained immediately after cross-linking. Figure 10 shows the methylene blue adsorption of distinct types of alginate and mixed gels cross-linked with Ca 2+ ions.Figure 11 presents the methyl orange sorption of chitosan gels.

Effect of the Physical State of Gels on the Dye Sorption
It is obvious that the dried cryo-and xerogels can adsorb dyes almost equally (Figures 8 and 9).We were curious to compare the dye uptake of the hydrogels with that of the dried gels, so we also performed dye uptake studies with the gels obtained immediately after cross-linking. Figure 10 shows the methylene blue adsorption of distinct types of alginate and mixed gels cross-linked with Ca 2+ ions.Figure 11 presents the methyl orange sorption of chitosan gels.

Effect of the Physical State of Gels on the Dye Sorption
It is obvious that the dried cryo-and xerogels can adsorb dyes almost equally (Figures 8 and 9).We were curious to compare the dye uptake of the hydrogels with that of the dried gels, so we also performed dye uptake studies with the gels obtained immediately after cross-linking. Figure 10 shows the methylene blue adsorption of distinct types of alginate and mixed gels cross-linked with Ca 2+ ions.Figure 11 presents the methyl orange sorption of chitosan gels.The equilibrium methylene blue uptake was close to 95% for all three types of alginate gels, and it was slightly lower (around 80-85%) for the mixed gels.The results in Figure 10 clearly show that the dye uptake of hydrogels is practically the same as that of cryo-and xerogels.This means that the drying process can be omitted for gels prepared for application in water purification.In addition, it is essential to note that the storage of gels is easier in dried form, and the xerogels can be produced by a cheaper and simpler method than freeze-drying.
There is already some variation in the dye uptake of the different chitosan gels.The hydrogels can bind with 5-10% less methyl orange dye than the dried gels.As previously described, by lowering the pH of methyl orange solution, the dye sorption of chitosan gels is likely to be increased.

Comparison of the Present Gel Sorptions with Results of the Literature
In the Section 1, we pointed out that the alginate and chitosan adsorbents (gels) are often compared in terms of the specific dye uptake.The specific dye uptake values for our gels are shown in Table 2. Comparing the data with those published in the literature and given in Table 1, we find that while the specific methylene blue uptake of our alginatecontaining samples is much lower than that of most alginate adsorbents (Table 1), our gels can bind with cationic dye (methylene blue), over a given time, in roughly similar proportions (expressed as %) as the reference samples.The low specific capacity values can be explained by the relatively high mass of adsorbents used in the research.Only adsorbents containing nanofibers or nanofillers show better efficiency (Table 1).
For methylene blue, the dye adsorption capacity of the 50:50 sample [24], highlighted in Table 1 and prepared similarly to our sample, is lower than that of our cryogel, which could be explained by the adsorption measurement being performed at too low a pH.While our alginate-containing gels performed similarly well in cationic dye binding as the alginate-containing adsorbents reported in the literature, the anionic dye binding of the chitosan-based gels was found to be much lower than that of the reference samples.This can be clearly explained by the fact that the pH was not adjusted adequately during our measurement.The literature results show that high-efficiency methyl orange adsorption can be achieved in highly acidic media (pH = 3).The equilibrium methylene blue uptake was close to 95% for all three types of alginate gels, and it was slightly lower (around 80-85%) for the mixed gels.The results in Figure 10 clearly show that the dye uptake of hydrogels is practically the same as that of cryoand xerogels.This means that the drying process can be omitted for gels prepared for application in water purification.In addition, it is essential to note that the storage of gels is easier in dried form, and the xerogels can be produced by a cheaper and simpler method than freeze-drying.
There is already some variation in the dye uptake of the different chitosan gels.The hydrogels can bind with 5-10% less methyl orange dye than the dried gels.As previously described, by lowering the pH of methyl orange solution, the dye sorption of chitosan gels is likely to be increased.

Comparison of the Present Gel Sorptions with Results of the Literature
In the Section 1, we pointed out that the alginate and chitosan adsorbents (gels) are often compared in terms of the specific dye uptake.The specific dye uptake values for our gels are shown in Table 2. Comparing the data with those published in the literature and given in Table 1, we find that while the specific methylene blue uptake of our alginatecontaining samples is much lower than that of most alginate adsorbents (Table 1), our gels can bind with cationic dye (methylene blue), over a given time, in roughly similar proportions (expressed as %) as the reference samples.The low specific capacity values can be explained by the relatively high mass of adsorbents used in the research.Only adsorbents containing nanofibers or nanofillers show better efficiency (Table 1).
For methylene blue, the dye adsorption capacity of the 50:50 sample [24], highlighted in Table 1 and prepared similarly to our sample, is lower than that of our cryogel, which could be explained by the adsorption measurement being performed at too low a pH.While our alginate-containing gels performed similarly well in cationic dye binding as the alginate-containing adsorbents reported in the literature, the anionic dye binding of the chitosan-based gels was found to be much lower than that of the reference samples.This can be clearly explained by the fact that the pH was not adjusted adequately during our measurement.The literature results show that high-efficiency methyl orange adsorption can be achieved in highly acidic media (pH = 3).

Sample Preparation
During the sample preparation, alginate and chitosan solutions were prepared first.To the chitosan solution, 4 g of chitosan was dissolved in 100 mL of distilled water with the addition of 1 mL of (96%) acetic acid using gentle heating (40-50 • C) and stirring.A total of 4 g sodium alginate was dissolved in 100 mL distilled water and stirred for about 1 h on a magnetic stirrer at room temperature.At the end of the solution, a yellowish viscous solution was obtained in both cases.
A total of 10 mL of the prepared solutions were pipetted into plastic sample holders.Three different compositions were used for preparing gels: chitosan (100%), alginate (100%), and chitosan-alginate (50-50%).The samples were stored at −20 • C for 24 h.Then, the lyophilization was performed on a LaboGene ScanVac Superior XS type (Scientific Laboratory Supplies, Notthingam, UK) device at −100 • C, at a pressure of 0.276 mbar, for 72 h.The samples were dry and porous (Figure 12) as their water content was sublimated.

Sample Preparation
During the sample preparation, alginate and chitosan solutions were prepared first.To the chitosan solution, 4 g of chitosan was dissolved in 100 mL of distilled water with the addition of 1 mL of (96%) acetic acid using gentle heating (40-50 °C) and stirring.A total of 4 g sodium alginate was dissolved in 100 mL distilled water and stirred for about 1 h on a magnetic stirrer at room temperature.At the end of the solution, a yellowish viscous solution was obtained in both cases.
A total of 10 mL of the prepared solutions were pipetted into plastic sample holders.Three different compositions were used for preparing gels: chitosan (100%), alginate (100%), and chitosan-alginate (50-50%).The samples were stored at −20 °C for 24 h.Then, the lyophilization was performed on a LaboGene ScanVac Superior XS type (Scientific Laboratory Supplies, Notthingam, UK) device at −100 °C, at a pressure of 0.276 mbar, for 72 h.The samples were dry and porous (Figure 12) as their water content was sublimated.For "green" cross-linking, 30 mL of sodium citrate (for chitosan) or calcium chloride (for alginate) solution at a concentration of 2 g/100 mL was used.The mixed gel was prepared with both cross-linking agents.In each case, the cross-linking was carried out for 20 min, and the hydrogel was washed several times with distilled water and dried in two ways.The xerogels were prepared by drying the samples for one week in a phosphorus pentoxide desiccator.The cryogels were prepared by freeze-drying in the same way as was introduced above.For "green" cross-linking, 30 mL of sodium citrate (for chitosan) or calcium chloride (for alginate) solution at a concentration of 2 g/100 mL was used.The mixed gel was prepared with both cross-linking agents.In each case, the cross-linking was carried out for 20 min, and the hydrogel was washed several times with distilled water and dried in two ways.The xerogels were prepared by drying the samples for one week in a phosphorus pentoxide desiccator.The cryogels were prepared by freeze-drying in the same way as was introduced above.

Characterization
The structure of the dry samples was studied with a scanning electron microscope (SEM).A JEOL JSM-6380 LA-type (Jeol Ltd., Tokyo, Japan) piece of equipment was used for the tests.The non-cross-linked samples and the xero-and cryogels were examined both longitudinally and transversely.
To determine the swelling of the samples, dry samples of known weight (W1) were placed in distilled water for 20 min.After removal from the water, the excess water remaining on the surface of the hydrogel was gently wiped off with filter paper, and the mass of the swollen samples (W2) was measured.The swelling rate was calculated from the measured masses using the following formula: The dye uptake of the gels was investigated in solutions with a concentration of 300 mg/L.Measurements were also performed in solutions with a concentration of 40 mg/L to demonstrate the dye uptake qualitatively.Methylene blue and methyl orange dyes were selected for characterizing gels' cationic and anionic dye sorption, respectively (Figure 13).

Characterization
The structure of the dry samples was studied with a scanning electron microscope (SEM).A JEOL JSM-6380 LA-type (Jeol Ltd., Tokyo, Japan) piece of equipment was used for the tests.The non-cross-linked samples and the xero-and cryogels were examined both longitudinally and transversely.
To determine the swelling of the samples, dry samples of known weight (W1) were placed in distilled water for 20 min.After removal from the water, the excess water re maining on the surface of the hydrogel was gently wiped off with filter paper, and the mass of the swollen samples (W2) was measured.The swelling rate was calculated from the measured masses using the following formula: The dye uptake of the gels was investigated in solutions with a concentration of 300 mg/L.Measurements were also performed in solutions with a concentration of 40 mg/L to demonstrate the dye uptake qualitatively.Methylene blue and methyl orange dyes were selected for characterizing gels' cationic and anionic dye sorption, respectively (Figure 13).A total of 50 mL of dye solution was poured onto 0.2 g of gel (dry weight), and sam ples were taken from the solution at specified intervals.The absorbance of the samples was measured at 464 (MO) and 664 (MB) nm using a Unicam UV-VIS UV 500 spectropho tometer (Mettler-Toledo Ltd., Budapest, Hungary).From the absorbance values, the actua concentration of the dyesolutions was calculated using calibration equations to determine the amount of dye bound in the gel under test at a given time (Equations ( 2) and ( 3)).

𝑐 = 𝑐 𝑀 𝑥
(2 where ct is the mass concentration of the dye at time t (mg/L), csp is the concentration de termined from the absorbance measured with the spectrophotometer (mmol/L), Mdye is the molar mass of the dye (mg/mmol = g/mol), and x is the dilution (−) used during sample preparation.where qt and qe are the dye sorption capacity (mg/g) at time t and at the end of the meas urement, ci and cf are the initial and final concentration of the dye solution (mg/L), ct is the concentration measured at time t (mg/L), Vi is the initial volume of the dye solution (L) ma is the dry sample weight (g) of the adsorbent, R (%) is the dye removal efficiency.A total of 50 mL of dye solution was poured onto 0.2 g of gel (dry weight), and samples were taken from the solution at specified intervals.The absorbance of the samples was measured at 464 (MO) and 664 (MB) nm using a Unicam UV-VIS UV 500 spectrophotometer (Mettler-Toledo Ltd., Budapest, Hungary).From the absorbance values, the actual concentration of the dyesolutions was calculated using calibration equations to determine the amount of dye bound in the gel under test at a given time (Equations ( 2) and ( 3)).
where c t is the mass concentration of the dye at time t (mg/L), c sp is the concentration determined from the absorbance measured with the spectrophotometer (mmol/L), M dye is the molar mass of the dye (mg/mmol = g/mol), and x is the dilution (−) used during sample preparation.
where q t and q e are the dye sorption capacity (mg/g) at time t and at the end of the measurement, c i and c f are the initial and final concentration of the dye solution (mg/L), c t is the concentration measured at time t (mg/L), V i is the initial volume of the dye solution (L), m a is the dry sample weight (g) of the adsorbent, R (%) is the dye removal efficiency.

Figure 6 .
Figure 6.Swelling of the alginate-chitosan gels as a function of chitosan content.

Figure 6 .
Figure 6.Swelling of the alginate-chitosan gels as a function of chitosan content.

Figure 8 .
Figure 8. Dye adsorption of cryo-and xerogels from a 300 mg/L methylene blue solution.

Figure 10 .
Figure 10.Dye adsorption of alginate and mixed xero-, cryo-and hydrogels from a 300 mg/L methylene blue solution.

Figure 9 .
Figure 9. Dye adsorption of cryo-and xerogels from a 300 mg/L methyl orange solution.

Figure 9 .
Figure 9. Dye adsorption of cryo-and xerogels from a 300 mg/L methyl orange solution.

Figure 10 .
Figure 10.Dye adsorption of alginate and mixed xero-, cryo-and hydrogels from a 300 mg/L methylene blue solution.

Table 2 .
Methylene blue sorption of alginate and alginate-chitosan (50-50) mixed gels and methyl orange sorption of chitosan gels measured for 800-850 h.The effect of gel physical state on the specific dye uptake capacity and dye removal efficiency.

Table 2 .
Methylene blue sorption of alginate and alginate-chitosan (50-50) mixed gels and methyl orange sorption of chitosan gels measured for 800-850 h.The effect of gel physical state on the specific dye uptake capacity and dye removal efficiency.